If you live and train at sea level, you will likely find it especially difficult to maintain your typical sea-level race pace at high altitude. The partial pressure, or relative concentration, of oxygen at high altitude is lower than at sea level, leading to a series of physiological responses that reduce your body's ability to supply working muscles with oxygen. This will have a negative impact on your endurance running performance until your body adjusts to the elevation. It takes your body about three to six weeks to fully acclimate to high altitude. However, your endurance race times at high altitude will never be as fast as they would be at sea-level.
The low partial pressure of oxygen at altitude leads to pulmonary, blood and muscle changes during the initial weeks of altitude living. Within a few days of ascending, your breathing rate at rest and during exercise increases in an effort to get more oxygen to your body tissues. Additionally, the lack of oxygen stimulates erythropoietin release from the kidney, which increases red cell production. Red blood cells contain oxygen-carrying hemoglobin, so the increased red blood cell count helps transport more oxygen to various body tissues. Altitude exposure increases muscle capillary density, which allows the circulation to deliver more oxygen-rich blood to the muscles. Prolonged altitude exposure also decreases muscle mass, although the mechanisms for this are not clear.
Exercise Performance at High Altitude
Maximal oxygen uptake decreases significantly at elevations above 5,000 feet. This is due to decreased maximal cardiac output -- a function of heart rate and blood volume per heartbeat -- along with reduced concentration of oxygen in your arterial blood. Although maximal oxygen uptake at altitude may improve with several weeks of altitude training, it will never reach your sea-level oxygen uptake capacity. Furthermore, you cannot race at the same volume and intensity that you could at sea level, so long-term training at high altitude may reduce your race performance.
Time Course of Acclimatization
Distance running performance will be lower during the first several days at high altitude. Your work capacity is reduced, and the combination of low oxygen and high carbon dioxide in your body tissues may lead to acute altitude sickness. You are also more likely to become dehydrated as you adjust to high altitude, further limiting performance capacity. Low water-vapor pressure at high altitude increases sweat evaporation and you lose more water via an increased breathing rate. Symptoms typically subside after about a week, but your cardiopulmonary system takes longer to fully adapt, usually at least two weeks. A 2005 study published in "International Journal of Sports Medicine" showed increased hemoglobin mass and red blood cell volume after three weeks at altitude, suggesting significant altitude acclimatization.
Optimal Race Performance at High Altitude
If you live at sea level and have a race at high altitude, you should train at high altitude for at least two weeks, though preferably three to six weeks, before your competition. Dehydration and acute altitude sickness symptoms are more likely to occur within the first one to two weeks of altitude exposure, and will likely worsen race performance. However, since these symptoms often take 24 to 48 hours to develop, you may also wish to race within 24 hours of your arrival at altitude. Although you will not acclimate to the altitude during this short time frame, you likely will avoid the physical symptoms of altitude exposure.